It has been desired in magnetic recording media for high density recording that the surface properties of the magnetic layer be improved to reduce spacing loss between the medium and a magnetic head. For this purpose, not only do the surface properties of the magnetic layer have to be improved by improving the manufacturing technology of the magnetic layer but also the surface properties of the support have to be improved. It has been attempted to make the thickness of the magnetic layer thinner in order to reduce thickness loss because the recording wavelength is lessened with increased recording density. As a result, the surface properties of the magnetic layer are more affected by the surface properties of the support.
However, the following reasons limit improvements in the surface properties of a support which is used for magnetic recording media. That is, as the surface properties of a film used as a support for a magnetic layer become better, frictional resistance to traveling rolls increased, and, as a result, in winding the film on the traveling rolls, the film meanders or is wrinkled. Further, the film or web cannot be uniformly wound because frictional resistance between opposite surfaces of the film to be wound increases.
Accordingly, it is preferred in the film manufacturing step that the surface of the film have a surface roughness not less than 0.01 .mu.m, but it is required, to provide a magnetic layer having better surface properties suitable for high density recording on a film support, that the surface of film have a surface roughness less than 0.01 .mu.m.
Various approaches have been proposed to overcome these inconsistent factors. For example, it is suggested in Japanese Patent Application (OPI) No. 109605/78 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") that a magnetic layer be coated on a support on which stilted fine particles of a thermoplastic resin have been provided and removed by dissolution with a solvent. However, according to this method, the characteristics necessary for magnetic recording media for high density recording cannot be attained.
To overcome the problems which have not been improved by conventional methods, the present inventors proposed in U.S. patent application Ser. No. 657,991, filed Oct. 5, 1984 that an intermediate layer containing a compound polymerizable by exposure to radiation such as electron beams or ultraviolet rays be coated on a non-magnetic support having a surface roughness not less than 0.007 .mu.m be exposed to radiation to polymerize and harden the same, and then a magnetic layer is coated on the intermediate layer.
The above problems are overcome by such a magnetic recording medium. That is, surface properties are improved because the coated intermediate layer is leveled before it is hardened by radiation exposure, and, therefor, the surface properties of the magnetic layer which is coated on the intermediate layer after the intermediate layer is hardened are also improved. Accordingly, even if a support having a surface roughness not less than 0.01 .mu.m is used, a magnetic recording medium having better surface properties which is useful for high density recording can be obtained.
The inventors continued to search for even more improved magnetic recording media and found that a magnetic recording medium having an intermediate layer exposed to radiation sometimes or often results in dropout due to the following reasons. That is, it is preferred, as described above, that the surface roughness (Ra) of film support be not less than 0.01 .mu.m, but protrusions whose height is 0.2 .mu.m or more (called "fish eyes") present on the film support cause dropout as the influence of such protrusions cannot be completely removed, even if a magnetic layer is provided after the intermediate layer is coated on the film support.
As a result of further research and development to remove the influence of the "fish eyes", the inventors found that to obtain a magnetic recording medium having improved surface properties which is not accompanied by the problem of "fish eyes" one can provide an intermediate layer on a support, which support has been subjected to an abrasion treatment or calendering treatment in advance, and then provide a magnetic layer on the intermediate layer.